The present invention relates to a noise removing method which is applicable to a sensing circuit with a measuring means for measuring the signal quantity of a sensing objective signal representing a physical quantity or the like. Furthermore, the present invention relates to a CMOS sensing circuit including a CMOS element serving as a measuring device.
A sensing circuit, used for detecting the physical quantity such as pressure or acceleration, usually comprises a signal processing circuit for processing a sensing objective signal sent from a sensor transducer. One of conventional circuit components incorporated in the signal processing circuit is a bipolar transistor which is characteristic in that the internal noise is relative low.
According to this sensing circuit, the internal noise of the bipolar transistor is sufficiently small compared with a sensing objective signal. The influence of a noise component involved in the sensing objective signal is small and not troublesome. However, recent sensing circuits are required to reduce the cost, downsize the circuit scale, and smarten the sensing functions. For example, self diagnostic function and self correcting function are required for enhancing the intelligence of the sensing circuit. Furthermore, improvement of the digital signal processing function is required. Such recent requirements cannot be satisfied by a circuit arrangement including the bipolar transistor. It is therefore necessary to arrange the circuit as a CMOS circuit using a CMOS element which is capable of increasing the degree of integration.
In such a case, the sensing objective signal is processed by a measuring circuit. An interface, such as a CMOS amplification circuit (i.e., analog amplifier), is generally provided as a pre-stage interface for the measuring circuit. The measuring circuit is constituted by a CMOS element. The MOS transistor, constituting the CMOS amplification circuit, is subjected to 1/f noise which is very large. Thus, the internal noise (i.e., low-frequency noise) of the MOS transistor is large. The offset noise of the CMOS amplification circuit becomes large. The offset noise is the low-frequency noise of approximately 10 Hz or below, which overlaps with the frequency band of the sensing objective signal. This makes it difficult to remove the offset noise by using a filter circuit (i.e., a low-pass filter).
As described above, the CMOS sensing circuit is inevitably subjected to the offset noise involved in its signal processing system. The offset noise fluctuates at a low frequency. Meanwhile, a general sensing objective signal fluctuates at a low frequency too. When the offset noise is larger than the minimum level of the sensing objective signal, a finally obtained sensing output is greatly influenced by the noise. As a result, it becomes impossible to maintain the sensing accuracy at a required or satisfactory level. In addition, the offset voltage existing in a signal amplification circuit varies due to the aging. In this respect, this offset voltage is a sort of extra low-frequency noise. Similar problem will arise.